Originally Posted by
Bike Gremlin
To quote Brandt:
"With tension, wires can support compression loads up to the point where they become slack.
My problem with that quote is that there is no compressive load on the wire spoke. There’s no need to even discuss compressive loads on wire spoked wheels. I’ll agree that the spoke is elongated through tension and it shortens with loading of the rim
because of a reduction in tension. The rim is compressed but the spokes don’t experience compression of any kind. Brandt is looking at compression and tension as they are simply forces with the opposite sign. Again, that is not the case.
Consider what happens if you loosen the nipple on a tensioned spoke. Tension decreases as the nipple is loosened. Is the spoke undergoing compression? Obviously not. When the rim deforms under compression only the rim experiences compression. The spoke experiences a decrease in tension. Of course the other spokes pick up that decrease by increasing in tension.
The same loads that increase compression in wooden spokes, reduce tension in wires.
Yes, but... A wooden spoked wheel (or car wheel) is compressed by the weight of the vehicle. The rim is compressed...I prefer to say deformed...by the weight of the vehicle. But that’s where the comparison ends. The wire spoke of a wheel doesn’t undergo the same compressive forces as the wooden spoke does. In a double wall bicycle rim, the wire spoke is hanging in the space between the walls of the rim. There is nothing pushing up on the spoke at all. This is why the load hangs from the rim at the top of the wheel instead of standing on the spokes. There’s nothing but air to
stand on.
As in a wooden-spoked wheel, the bottom spokes of a wire wheel become shorter under load, but instead of gaining in compression, they lose tension.
With the same load, the net change in force is the same for both wheels.
The algebraic sum of negative and positive forces (compression and tension) is the same."
The highlighted sentence is what I see as his biggest mistake. He is treating compression and tension as an algebraic sum as if when compression goes up, tension decreases. That is not the case. Think of the wooden spoked wheel. I agree that the bottom spokes of the wooden spoked wheel get shorter under load because they are being compressed. However, when the load is removed and the wooden spokes lengthen, they can’t be said to be lengthening due to an increase in tension because the wooden spokes are never in tension. They are only being decompressed. In other words, compression becomes zero but tension is also zero.
The opposite happens in a tensioned wire wheel. The spokes are lengthen through tension and when the rim deforms under load, the spokes shorten but that is because tension is decreasing. The wire spoke itself is never under a load other than tension. Tension goes back up as the rim is unloaded but compression on the spoke itself is zero.
In short, compression and tension aren’t added to find the force on the spokes of either a wooded spoke or wire spoke. The force on each is either compression or tension but not both. That holds for any discussion of compression and tension. An object can’t be pulled and pushed from the same point at the same time so compression and tension aren’t algebraical connected.
English is not my native, but to me this sounds like the same thing you said, and the same thing I said, only using different words.
Your English is excellent and far better than my ___(insert any language here
). My objection is to the use of the term compression (except when talking about the rim) with regard to a wire spoke. To compress something you have to have a force that is pushing on the object. There is nothing to
push on when it comes to a wire spoke.